This invention relates to rotary mechanisms of the trochoidal type for pumps, compressors, fluid motors, and internal combustion engines, and more particularly to a rotor and gear assembly for such mechanisms.
In mechanisms of this type the rotor is mounted for rotation on an eccentric portion of a shaft within a housing, the rotor also performing a planetary motion within the trochoidal housing. An internal ring gear is fixed on one side face of the rotor and engages a stationary spur gear surrounding the shaft to assist in maintaining phasing between the rotor and its trochoidal housing. In early examples of the prior art the ring gear was simply bolted, pinned, or welded to the rotor. However, the materials of the gear and the rotor are different and have different coefficients of thermal expansion, as well as an interface between the two parts across which heat is not readily transferred. Such tight unions between gear and rotor do not allow for differential thermal expansion of the parts, either radially or axially, nor do they allow for cyclically varying stress loads on the gear. Therefore, breakages and distortions occurred, occasional variations in phasing, and faults in concentricity between the rotor and gear.
Various expedients have been tried to correct these conditions, such as flexible bolting and splining, but these arrangements are very expensive to fabricate and assemble. Resilient pins have been tried, as in U.S. Pat. No. 3,297,240, issued Jan. 10, 1967 to Tatsutomi. In that patent a plurality of split tubular pins were circumferentially spaced around the gear, parallel to the rotor axis and fitting tightly in bores in both the gear and the rotor. Under varying loading of the gear or differential expansion between the gear and the rotor, the pins will compress slightly owing to squeezing of the split tubes. However, no provision is made for axial retention of the gear, beyond the frictional fit of the pins, and it is possible for the gear to displace axially so that its entire side face can be in contact with the side wall of the mechanism, causing undue friction and excessive wear of the side wall, since the gear is formed of hardened material. Further, concentricity may not be maintained, since radial expansion of the rotor may be greater in one portion than another, displacing the gear axis transversely from the rotor axis.
The lack of axial retention is corrected by U.S. Pat. No. 3,619,092, issued Nov. 17, 1971 to Kurio. This patent employs the same axially parallel, split tubular pins, but the gear is retained axially by a snap ring fitting into an annular groove in the rotor, with a wedging action pressing the gear tightly against the rotor face. This again increases machining and assembly expense, and since the rotor has a higher coefficient of thermal expansion than the gear and receives more heat, axial expansion can cause the retaining groove for the snap ring to become axially wider, whereupon the ring expands radially outwardly by spring action to fill the groove. Upon cooling, the wedge surface is too flat to allow recompressing the ring radially inwardly when the groove dimension shrinks, so that distortion of the retaining lip of the groove may result. Also, there is still no provision for maintaining concentricity.
In U.S. Pat. No. 3,830,599, issued Aug. 20, 1974 to Poehlman, concentricity in the presence of differential radial expansion is achieved by the use of axially oriented solid pins fitting in radially slotted holes in the gear, which allows radial expansion of a portion of the rotor with respect to the gear, any single pin sliding in its slot without movement of the others and without displacing the gear axis transversely. However, there is again a delicate machining and assembly job to provide axial retention of the gear by means of special standoff bolts, and there is no provision for absorbing momentary circumferential shock loads on the gear caused by cyclic variations in loading.